A light-emitting device comprising a compound that exhibits light emission from a triplet excited state (hereinafter, also referred to as a “phosphorescence-emitting compound”) used as a light-emitting material for a light-emitting layer therein has been known to have high luminous efficiency. When the phosphorescence-emitting compound is used in a light-emitting layer, a composition comprising the compound added to a matrix is usually used as a light-emitting material. A polymer such as polyvinylcarbazole is used as the matrix, because a thin film can be formed by application (PATENT DOCUMENT 1).
However, such a polymer has the problem that the lowest unoccupied molecular orbital (hereinafter, also referred to as “LUMO”) is high, thus resulting in difficult electron injection. On the other hand, when a conjugated polymer such as polyfluorene is used as the matrix, electrons can be injected relatively easily because of its low LUMO. However, such a conjugated polymer has a small lowest triplet excitation energy and is therefore considered to be unsuitable particularly for use as a matrix for light emission with a wavelength shorter than green light emission (PATENT DOCUMENT 2). For example, a light-emitting material comprising a conjugated polymer polyfluorene and a triplet light-emitting compound (NON-PATENT DOCUMENT 1) has low luminous efficiency.    PATENT DOCUMENT 1: JP-A-2002-50483    PATENT DOCUMENT 2: JP-A-2002-241455    NON-PATENT DOCUMENT 1: APPLIED PHYSICS LETTERS, 80, 13, 2308 (2002)